1. Field of the Invention
The present invention relates generally to an evaporative type cooling system for an internal combustion engine wherein liquid coolant is permitted to boil and the vapor used as a vehicle for removing heat therefrom, and more specifically to such a system which features a double pump arrangement which simultaneously enables (a) coolant condesate to be returned to the coolant jacket and (b) rapid control of pressure prevailing in the cooling circuit so as to offset any undesirable effects on temperature control that sudden changes in engine operation and/or ambient conditions might have.
2. Description of the Prior Art
In currently used "water cooled" internal combustion engines such as shown in FIG. 1 of the drawings, the engine coolant (liquid) is forcefully circulated by a water pump, through a cooling circuit including the engine coolant jacket and an air cooled radiator. This type of system encounters the drawback that a large volume of water is required to be circulated between the radiator and the coolant jacket in order to remove the required amount of heat. Further, due to the large mass of water inherently required, the warm-up characteristics of the engine are undesirably sluggish. For example, if the temperature difference between the inlet and discharge ports of the coolant jacket is 4 degrees, the amount of heat which 1 Kg of water may effectively remove from the engine under such conditions is 4 Kcal. Accordingly, in the case of an engine having an 1800 cc displacement (by way of example) is operated full throttle, the cooling system is required to remove approximately 4000 Kcal/h. In order to achieve this, a flow rate of 167 liter/min (viz., 4000-60.times.1/4) must be produced by the water pump. This of course undesirably consumes a number of otherwise useful horsepower.
FIG. 2 shows an arrangement disclosed in Japanese Patent Application Second Provisional Publication No. Sho. 57-57608. This arrangement has attempted to vaporize a liquid coolant and use the gaseous form thereof as a vehicle for removing heat from the engine. In this system the radiator 1 and the coolant jacket 2 are in constant and free communication via conduits 3, 4 whereby the coolant which condenses in the radiator 1 is returned to the coolant jacket 2 little by little under the influence of gravity.
This arrangement has suffered from the drawbacks that the radiator, depending on its position with respect to the engine proper, tends to be at least partially filled with liquid coolant. This greatly reduces the surface area via which the gaseous coolant (for example steam) can effectively release its latent heat of vaporization and accordingly condense, and thus has lacked any notable improvement in cooling efficiency.
Further, with this system in order to maintain the pressure within the coolant jacket and radiator at atmospheric level, a gas permeable water shedding filter 5 is arranged as shown, to permit the entry of air into and out of the system. However, this filter permits gaseous coolant to readily escape from the system, inducing the need for frequent topping up of the coolant level.
A further problem with this arrangement has come in that some of the air, which is sucked into the cooling system as the engine cools, tends to dissolve in the water, whereby upon start up of the engine, the dissolved air tends to come out of solution and form small bubbles in the radiator which adhere to the walls thereof and form an insulating layer. The undissolved air also tends to collect in the upper section of the radiator and inhibit the convention-like circulation of the vapor from the cylinder block to the radiator. This of course further deteriorates the performance of the device.
European Patent Application Provisional Publication No. 0 059 423 published on Sept. 8, 1982 discloses another arrangement wherein, liquid coolant in the coolant jacket of the engine, is not forcefully circulated therein and permitted to absorb heat to the point of boiling. The gaseous coolant thus generated is adiabatically compressed in a compressor so as to raise the temperature and pressure thereof and thereafter introduced into a heat exchanger (radiator). After condensing, the coolant is temporarily stored in a reservoir and recycled back into the coolant jacket via a flow control valve.
This arrangement has suffered from the drawback that when the engine is stopped and cools down the coolant vapor condenses and induces sub-atmospheric conditions which tend to induce air to leak into the system. This air tends to be forced by the compressor along with the gaseous coolant into the radiator. Due to the difference in specific gravity, the air tends to rise in the hot environment while the coolant which has condensed moves downwardly. The air, due to this inherent tendency to rise, forms pockets of air which cause a kind of "embolism" in the radiator and which badly impair the heat exchange ability thereof.
U.S. Pat. No. 4,367,699 issued on Jan. 11, 1983 in the name of Evans (see FIG. 3 of the drawings) discloses an engine system wherein the coolant is boiled and the vapor used to remove heat from the engine. This arrangement features a separation tank 6 wherein gaseous and liquid coolant are initially separated. The liquid coolant is fed back to the cylinder block 7 under the influence of gravity while the relatively dry gaseous coolant (steam for example) is condensed in a fan cooled radiator 8.
The temperature of the radiator is controlled by selective energizations of the fan 9 which maintains a rate of condensation therein sufficient to provide a liquid seal at the bottom of the device. Condensate discharged from the radiator via the above mentioned liquid seal is collected in a small reservoir-like arrangement 10 and pumped back up to the separation tank via a small constantly energized pump 11.
This arrangement, while providing an arrangement via which air can be initially purged to some degree from the system tends to, due to the nature of the arrangement which permits said initial non-condensible matter to be forced out of the system, suffers from rapid loss of coolant when operated at relatively high altitudes. Further, once the engine cools air is relatively freely admitted back into the system. The provision of the bulky separation tank 6 also renders engine layout difficult.
Japanese Patent Application First Provisional Publication No. Sho. 56-32026 (see FIG. 4 of the drawings) discloses an arrangement wherein the structure defining the cylinder head and cylinder liners are covered in a porous layer of ceramic material 12 and wherein coolant is sprayed into the cylinder block from shower-like arrangements 13 located above the cylinder heads 14. The interior of the coolant jacket defined within the engine proper is essentially filled with gaseous coolant during engine operation at which time liquid coolant sprayed onto the ceramic layers 12.
However, this arrangement has proven totally unsatisfactory in that upon boiling of the liquid coolant absorbed into the cramic layers, the vapor thus produced and which escapes into the coolant jacket inhibits the penetration of fresh liquid coolant and induces the situation wherein rapid overheat and thermal damage of the ceramic layers 12 and/or engine soon results. Further, this arrangement is of the closed circuit type and is plagued with air contamination and blockages in the radiator similar to the compressor equipped arrangement discussed above.
FIG. 7 shows an arrangement which is disclosed in U.S. Pat. No. 4,549,505 issued on October 1985 in the name of Hirano. The disclosure of this application is hereby incorporated by reference thereto.
For convenience the same numerals as used in the above mentioned patent are also used in FIG. 7.
This arrangement while solving the problems encountered with the prior art has itself encountered the problem that it requires no less than four electromagnetic valves and a corresponding number of conduits in order to conduct the required coolant management during the various modes of engine operation. These valves are relatively expensive and the relatively large number of conduits tends to clutter the engine compartment.
In order to overcome this problem it has been proposed in copending U.S. Pat. application Ser. No. 751,536 filed on July 3, 1985 in the name of Hirano et al, to utilize an arrangement wherein two of the valves (134 and 156) of the FIG. 7 arrangement were replaced by a single three-way valve disposed in the coolant return conduit 132 at a location between the pump 136 and the coolant jacket 120.
This arrangement while greatly simplifying the valve and conduit arrangement via which communication between the reservoir and the cooling circuit per se of the engine and simultaneously enabling improved coolant control via the use of a reversible pump, has suffered from the problems that the three-way valve tends to be expensive and apt to jamming from time to time. Further, due to the inherent construction of the valve the discharge of the coolant return pump tends to be restricted. Accordingly, upon the whole system becomming heated to the point of being thermally saturated (such as tends to occur after prolonged high load operation) the coolant return pump is sometimes subject to a cavitation problem wherein vapor is generated in the pump chamber or chambers which vastly reduces the discharge thereof. This induces the serious problem that insufficient liquid coolant is returned to the coolant jacket and the level of coolant therein drops in a manner which invites localized dryouts and overheating.
One way of solving this problem is to introduce fresh cool liquid coolant into the system immediately upstream of the pump upon cavitation occuring. However, this inevitably varies the amount of coolant contained in the system and thus requires subsequent adustment at a latter time. Moreover, the number of valves and conduits is increased by this measure and as such the same drawback inherent with the FIG. 7 arrangement is encountered.
A further problem with the three-way valve type arrangement has come in that when the valve is set to return coolant to the coolant jacket it is impossible to adjust the amount of coolant in the radiator using the pump and valve. Under high load operation when boiling becomes particularly vigorous a substantial amount of coolant tends to "bump" and boil over to the radiator in liquid form. Under these circumstances the interior radiator becomes wetted and partially filled with liquid coolant and thus reduces the amount of "dry" surface area available for the coolant vapor to release its latent heat of evaporation at at time when the maximum heat exchange efficiency of the radiator is most important. In order to reduce this level the pump must be frequently energized with the three-way valve set to return the liquid coolant to the coolant jacket. However, under these conditions the above mentioned caviation problem is apt to occur and compound the tendancy for a liquid coolant shortage to occur in the coolant jacket. Simultaneously opportunities to pump coolant out of the system in a manner which drops the pressure and temperature therein are vastly reduced and thus a control dilemma is encountered.
Hence, a requirement to be able to maintain the coolant jacket safely filled with sufficient liquid coolant and to simultaneously manage the amount of coolant in the system for the purposes of temperature control, has come into existence.
It will be noted that the above mentioned patent application was not published prior the priority date of the instant application and as such does not constitute actual prior art. The above discussion has been made with the intent of clarifying the background of the instant invention and includes knowledge which is not known to those not directly connected with the instant patent application. The content of said application is hereby incorporated by reference thereto.